The child pokemon is always the same family as the female. So if you breed a female Venusaur with a male Charizard, the baby will be Bulbasaur.
For two pokemon to breed they need to be in the same egg group like Umbreon&Dragonite says. You could always try putting the two in daycare and seeing what happens. Save your game beforehand then reset if they can't breed. See my answer here for how to check: http://pokemondb.net/pokebase/8816/ditto-is-losing-his-manly-charm